A known, conventional method for casting crystalline silicon ingot, is described as follows. First, poly-silicon was charged as a raw material and melted in the highly-pure quartz crucible 23 in vacuum or inert gas atmosphere shown in FIG. 12 , and after melting down, the melt is charged into the mold 24 which is made of the highly-pure graphite. The silicon in a liquid state 25 in the mold 24 is cast in ingot form being pulled down in a very slow speed through the heated zone which is controlled in a fixed temperature, for the purpose of controlling the size of the grain of the cast ingot, by making the cooling rate as low as possible. This method is known as a Wacker Ingot Casting Method (WICP) and a sort of modified well-known Bridgman/Stockbarger method. Another known method for casting crystalline silicon ingot, is described below and is shown in FIG. 11. This method was known as Cold Crucible method (JP-A-64-53733). The cold crucible consists of several water-cooled copper segments which are separated from each other by narrow slides. The electro-magnetic field created by the induction coil penetrates through the slides and couples into the charged material, then the melted material is lifted up by the magnetic force and heated. In this FIG. 11, numerals 29, 30 and 31 indicate a feeder, resistant heater and silicon crystal respectively.
Electron beam bombarding melting method (EBM) shown in FIG. 10, has also been known as an another method for casting crystalline silicon ingot. Electron beam is able to heat the melt quietly without causing any movement in molten metal 33 because electrons are accelerated in a high voltage field and bombarded to the target to heat the object. The advantage of the EBM method is to control the energy distribution within any shape of the scanning area of the ingot 34. In this FIG. 10, a numeral 35 indicates a water cooled crucible.
In the WICP method, material cost becomes very high because the highly pure materials should be used for crucible and mold, and the contamination from the de-molding flux can be expected in a high level. The maximum size of the cast ingot can be less than 100 kg, because of the controlling of the temperature of the mold, and of the limit of the effect of the removing of the impurities from the ingot by the effect of the concentration of the impurities from the solution in the ingot. The limitation of the ingot size would cause some problem for the increase of the size of the apparatus for the future expansion of the product. Above-mentioned "cold crucible-continuous casting method", however, is not only very difficult to operate but also consumes a considerable power for maintaining the melt and cooling the cold crucible. The melt is also stirred very much by electromagnetic magnetic force which may influence slightly the crystal growth during the solidification. The inner diameter of the coil has some limits concerning the electromagnetic force, and it may be difficult to cast the same size of WICP's ingot. From this point, "cold crucible-continuous casting method" has to be developed still more to meet the demand of production in large scale.
And, EBM has also a problem in casting operation. In general, the casting process of EBM is discontinuous in the ingot pulling down method. It is very difficult to maintain the casting operation because the first part of the ingot which solidifies is always contacting the water-cooled copper crucible, which is different from "cold crucible-continuous casting method", and the casting face is cooled down at a rapid speed so that stress is always applied to the ingot forming cracks, and the cast ingot is broken off.